1. Field of the Invention
The invention relates to an automobile and method for the automobile, and more particularly to an automobile and method for the automobile provided with an internal combustion engine capable of outputting power to a drive shaft.
2. Description of the Related Art
As an example of this kind of automobile, an automobile is proposed which is running utilizing power from an internal combustion engine and power from an electric motor, and in which a part of the power from the internal combustion engine is converted into electric power so as to charge a battery. In this automobile, the internal combustion engine and the electric motor are controlled such that while outputting power equivalent to a power requirement to a drive shaft, a charge amount of the battery increases when the automobile is in such a running state that an increased amount of charged electric power of the battery per increase amount of a unit fuel of the internal combustion engine increases. Accordingly, it is possible to reduce fuel consumption of the internal combustion engine with respect to the charge amount of the battery, thereby improving fuel economy, compared to an automobile that controls the charge amount of the battery merely in accordance with a state of charge SOC of the battery.
However, with an automobile such as disclosed in Japanese Patent Application Laid-Open No. 2001-298805, the internal combustion engine is sometimes operated in a low output region where operation efficiency (i.e. fuel economy) of the internal combustion engine is low, although the operation efficiency depends to some extent on a capacity and performance of the electric motor to be used. That is, when the output in the low output region where the operation efficiency is low cannot be supplemented by the output from the electric motor, the operation efficiency of the internal combustion engine may further decrease. Further, it is likely that the efficiency change with respect to the output change is large in the low output region of the internal combustion engine. Therefore, when operating the internal combustion engine while taking into consideration the electric charge to the battery, the output change of the internal combustion engine becomes greater, and thus the efficiency change of the internal combustion engine becomes greater. This may lead to unstable fuel consumption.